Food and lifestyle choices are the #1 most important thing when it comes to avoiding damage from covid-19. Far more important then any doctor or hospital or vaccine or anything like that.
In the USA we have nearly 180,000 people died from covid out of 330,000,000 people. Comorbidity play a huge rule in this. Comorbidity means that you have one or more additional serious diseases in addition to whatever disease is being discussed, in this case covid-19.
Out of those only about 6% are from covid alone. The rest (94%) died with had a average of 2.6 commodities. 71% had 2 or more. Most of the people who died who did not have comorbidities were already of advanced age.
Out of healthy younger individuals the total number of people that died is probably well below 2000. Across a entire population of nearly 327 million people.
Now the majority of the commodities are going to be respiratory issues. Things like pneumonia, ARDS, and the like. Which are heavily complicated by covid, of course. But circulatory diseases play a big role... things like hypertension and heart attacks. As does obesity and diabetes. Heart and obesity-related diseases are ones that people have control over. They are mostly a result of lifestyle choices and diet.
Which means, generally speaking, most people that die from covid do not die from only covid. They have died from getting covid combined with other diseases acquired after decades of shitty diet and no exercise.
If you want to enhance your chances of survival the most important things to do are going to be to make sure to eliminate additional sugars and refined grains from your diet, eat whole foods, get exercise, get lots of sun and make sure you have lots of vitamin-D. Vitamin-D is important for healthy immune system, but most Americans run with severe deficiencies. Sugars from whole fruits and vegetables are fine since they are consumed with a great deal of dietary fiber which negates the bad effects of sugar. But juice drinks and sodas should be considered inherently poisonous.
Due to the anti-fat craze of the latter half of the 20th century low-quality low-cost highly processed factory foods have eliminated most of the fat from their products to look good on paper. However low-quality foods taste like cardboard without the fat. So in order to make things taste decent they add as much sugar as possible. Usually in the form of fructose syrup from corn, which is sweeter and cheaper then cane sugar.
It's to the point were anything labelled 'low fat' usually means large amounts of sweetener. Avoid anything labelled 'low-fat' like the plague unless the food is naturally low-fat. A example of this badness is low-fat yogurt. If you like fruit yogurt buy whole milk yogurt and mix in your own chopped up fruit, much better.
(Whole milk Greek yogurt is often the best because it contains the least amount of sugars. But you have to pay attention to the labels)
It is this hormonal response to massive fructose (and other sugars, but fructose mainly) intake that causes obesity and related diseases. It's not a issue of calories-in versus calories-out since the type of food you eat changes the way the body responds to calories. It impacts your resting metabolism, the rate that fat is produced versus consumed, and many other things that cannot be controlled directly through exercise/physical activity.
I don't think it's really critically important what diet you pick.. whether it's Paleo, Mediterranean, Keto, Vegetarianism, or whatever. The important thing is creating disciplined structure around food that eliminates highly processed food in favor of a nutritionally balanced diet.
It's possible to have a incredibly shitty diet if you are vegetarian if you eat nothing but breads and pasta. But vegetarians can have very excellent diet provided they are careful about nutrition and stick to unprocessed foods. It's possible to have a nutritionally balanced meat-based diet since meat has much higher nutritional density then plants, but eating at McDonalds every day is a death sentence even if you throw away the buns.
This is why I think that the specifics don't matter a whole lot compared to simply picking a system that eliminates processed foods.
The minimal number of carbs and sugars that is necessary for a healthy diet is 0. Very literally: ZERO. You don't need them. They only provide calories. Nothing in the body depends on them. The glucose you need can be metabolized from other types of food.
Proper nutrition from whole foods and exercise is going to protect you more then any doctor or vaccine could possibly do for you.